The present invention relates to a radio-frequency (RF) antennae (single coils and multiple coil arrays) and, more particularly, to a novel method for providing multiple coaxial cable connections to an RF antenna without requiring the use of balanced-to-unbalanced transformers (baluns) at each connection.
It is well known to select an RF antenna to provide a desired RF current distribution which produces a desired RF magnetic field in a particular apparatus, e.g. in a nuclear magnetic residence (NMR) imaging system. Many types of desirable antennae, such as a quadrature volume imaging coil or a multiple surface coil array, require a plurality of separate input/output connections. Normally, each separate RF connection is a balanced connections. If an unbalanced, first coaxial cable is attached directly to the structure, the shield side of the coaxial cable is normally at ground potential, so that additional connections cannot be in coaxial form, because of the impossibility of providing a second shield attachment point which is a ground potential node, relative to the same ground potential at the first cable shield attachment point. Accordingly, the first coaxial cable attachment and/or the second coaxial cable attachment must be made to the RF antenna structure by means of an isolating circuit, such as a RF isolation transformer (a balun and the like), which prevents common mode currents from flowing while allowing passage of differential currents. Unfortunately, isolation transformers and/or baluns add at least one additional circuit element to the RF coil structure and, because these isolation components may have high voltages across their internal electronic components, place additional demands upon component tolerances, reliability, cost, and/or performance capability. It has been suggested to utilize inductive driving circuitry, including mutual-inductance coupling, to avoid direct multiple connections to the coil or coil array. Use of inductive drive is disadvantageous in that extra resonant structures must be manufactured and can each produce additional fields which may substantially add, in a potential deleterious fashion, to the RF fields already present within the NMR imaging volume. It is therefore highly desirable to provide a method for forming a plurality of RF connections to an RF antenna (a single coil or a coil array) in each of which connections a coaxial cable is connected directly to a different part of the antenna structure.